In the North Atlantic region high frequencies of tropical cyclones are found in the months of August, September and October. The period from December to May is free from cyclones. In the southwestern North Pacific greatest frequency is seen from July to October.
There are no cyclones from February to April in this region. Annual variation on the north Indian Ocean is rather complex. The approach of the summer monsoon marks the beginning of the hurricane season.
Maximum number of cyclones develops in late June or early July. It is interesting to note that during the peak period of the summer monsoon, the frequency of cyclones is comparatively lower.
It again increases in early autumn. In the southern hemisphere the late summer and early autumn record the maximum number of cyclones.
There are no hurricanes in the South Atlantic. The most plausible explanation for the absence of tropical cyclones in this region seems to be that the inter-tropical convergence zone remains to the north of the equator so that no weak tropical disturbance develops over this ocean. Besides, sea-surface temperature is relatively lower in the low latitudes.